Average tenancy and rent are not independent variables here. At very high market rates, I don't doubt tenancy lengths aren't that long; everyone is looking for

Average tenancy and rent are not independent variables here. At very high market rates, I don't doubt tenancy lengths aren't that long; everyone is looking for the next best deal. But a dramatic drop in rent will result in someone locking in that rate and literally never leaving. Taking that apt off the market essentially forever. I know a few people with rent controlled apartments in SF who have held them for decades so their rent is impossibly low. They'll never leave. At least one hasn't even lived in SF for many years now but the rent is so low it's worth it to them to keep it as an occasional weekend crash pad even though they live elsewhere.